A Beginner's Guide to Geocoding and Reverse Geocoding

Geocoding is the process of taking a text-based description of a location (like a street address or a place name) and converting it into spatial coordinates (latitude and longitude). It is one of the most common operations in GIS.

What is Geocoding?

If you have a spreadsheet with 1,000 customer addresses, you cannot map them until you know where they are. Geocoding translates those addresses into X/Y coordinates so they can be plotted on a map.

What is Reverse Geocoding?

Reverse geocoding is the opposite. You start with a set of coordinates and ask, "What is the address of this location?"

This is useful for applications like tracking a vehicle or analyzing a GPS point collected in the field.

How Geocoding Works

Geocoding relies on a "Locator" (or Geocoder) service. These services, like the Esri World Geocoder or Google Maps API, contain massive databases of road networks and address ranges.

1. You submit an address string (e.g., "1600 Pennsylvania Ave NW, Washington, DC").
2. The locator parses the string into components (street number, street name, city, state).
3. It matches the components to its database and interpolates the position along the street segment.
4. It returns a coordinate and a "score" indicating the confidence of the match.

Geocoding in GIS

Most GIS software has built-in geocoding tools:

• QGIS: Uses the "Geocoding" plugin or the batch geocoding tools in the processing toolbox.
• ArcGIS Pro: Uses the "Geocode Addresses" tool, which connects to the Esri World Geocoder or a local address locator.
• Online Tools: Many free web tools allow you to paste a list of addresses and download a GeoJSON file.

The Importance of Match Scores

Geocoding is not always perfect. The locator will return a score from 0 to 100. A score of 100 means an exact match. A score of 80 might mean it found the street but interpolated the house number.

Summary

Geocoding converts text into maps, and reverse geocoding converts maps into text. Understanding these processes is essential for any GIS professional working with address data.